(a) Field
The subject matter disclosed generally relates to fiber optics. More precisely, it relates to fiber Bragg grating sensors.
(b) Related Prior Art
Fiber Bragg gratings (FBGs) have proven to be useful devices for monitoring physical parameters including, but not limited to, temperature, strain, pressure, vibrations. FBGs are fabricated by inducing a permanent periodic refractive index change in the core of an optical fiber over a given length, typically a few millimeters. Such devices reflect light propagating in the optical fiber over a narrow wavelength range. The reflection spectrum typically has a narrow peak centered at a wavelength equal to twice the spatial period of the grating multiplied by the refractive index of the fiber. The width of the reflection spectrum is inversely proportional to the grating length. To use the FBG as a sensor, a transducing arrangement either changes the temperature or stretches the section of fiber containing the grating, both effects resulting in a change of the effective period of the grating, and therefore of the central wavelength of the reflected light. A measurement of that central wavelength can thus be correlated with the parameter being measured (the measurand). Such measurement can be made remotely by launching light into the optical fiber at a distance, and detecting and analyzing the spectral characteristics of the reflected light. Since the transmission loss of optical fiber is very low, the measurement apparatus can be located far away from the FBG sensor.
One advantage of FBGs as sensor devices is that the optical fiber is immune to electromagnetic interference, making possible their use in environments where electrical or electronic sensors would not perform well.
However, one of the main advantages of FBGs is that, due to their narrow spectral signature, multiple FBGs of different periods can be written at different locations along a single strand of optical fiber, and each FBG can be identified by its spectral signature. Thus multiple spatial locations can be sensed simultaneously with a single optical fiber by measuring the peak wavelength of the FBG's. Such a technique to integrate multiple FBG sensors with various central wavelengths along one optical fiber is termed “Wavelength Division Multiplexing”, or WDM.
Typically, the method used to monitor the central wavelength of the FBG sensors is to use a tunable optical source that repeatedly scans a broad wavelength range. The reflected light is collected via an optical circulator or an optical coupler located between the source and the FBGs, and then sent onto a photodetector. The peaks in the reflected light therefore correspond to the peak reflected wavelength of each FBG. Alternately, a broadband optical source can be used, with the central wavelengths of all the FBG sensors falling within the source spectrum. The reflected light is then sent into a spectrometer, and the reflected peaks are determined by analyzing the spectrometer data.
For WDM interrogation, all the FBG sensors along the optical fiber are made to have central wavelengths within the spectral range of either the tunable source or the broadband source. On the other hand, the wavelength spacing between each FBG must be large enough to account for the maximum spectral shift of each FBG under the influence of the parameter to be measured, so that there is no possibility of overlap between individual spectra, which would lead to ambiguous or erroneous measurements. One can therefore define a “spectral window”, which is the range of central wavelengths that one FBG sensor is likely to span given the range of the measurand. As a consequence, the maximum number of FBG sensors that can be interrogated by a given optical source is approximately equal to the spectral width or span of the source divided by the spectral width of the spectral windows of individual sensors.
For example, the central wavelength of a typical FBG will shift in proportion to strain. Thus if the strain to be measured is as high as 10,000με, the maximum wavelength shift would be approximately 10 nm for a FBG with a central wavelength of 1550 nm. If the tunable source has a range of 60 nm, as is typical of commercial sources, then the maximum number of sensors that can be interrogated with that source is 6, given by 60 nm divided by a spectral window of 10 nm.
Some applications would benefit from a much larger number of sensors in the same fiber. For example, spatially resolved temperature or strain measurements on pieces of equipment such as wind turbine blades, or generator rotors, would benefit from having a number of sensors in the range of 100 or more.
One way of augmenting the number of sensors is to use the time domain as well as the wavelength domain to discriminate between sensors. Such schemes are labeled “Time-division-multiplexing”, or TDM. In a TDM scheme, the light source used to interrogate the gratings is pulsed, and the pulse duration is made to be shorter than the time delay for light travelling from one grating to the next one. If the maximum reflectivity of each grating is small enough (typically 1%-2%), then only the first reflection from each grating is significant, and the reflected signal coming from light that has bounced multiple times on the grating reflectors is negligible. Therefore each pulse generates a series of echoes that can be temporally discriminated. As with WDM interrogation schemes, the source can either be tunable or broadband. The difference is that the detection apparatus must have a response time that is short enough to differentiate the reflections from the various gratings. Therefore, multiple FBG sensors can be used within a single spectral window. The total number of sensors is thus that of the WDM arrangement multiplied by the number of sensors that can be temporally discriminated.
Even though such combination TDM-WDM interrogation schemes can use tunable sources, most of the proposed prior art uses broadband optical sources. That is because the broadband source has all the wavelengths present at all times to interrogate the sensors, which means that the response time is not limited by the source. A tunable source always takes some time to scan the entire wavelength span, and that time ultimately limits the response time of the system.
When using a broadband optical source, the central wavelength of the grating sensors is most often determined by processing the reflected light with the use of spectrally sensitive filters, such as unbalanced interferometers, or ratiometric arrangements.
Prior art describing TDM and WDM sensing apparatus include patent publications WO2013001268, CA2379900A1, US20100128258, WO2004056017, all of which use a broadband optical sources, and various arrangements to gate the reflected signals, and detect their wavelengths.
Niewczas (WO2013001268A2) describes a system using a broadband light source, and interferometers to measure the wavelength of the reflected light. Cooper and Smith (CA2379900A1) also use a pulsed broadband optical source, and an optical modulator to gate the reflected pulses. Volanthen and Lloyd (US20100128258) also use a pulsed broadband source.
Everall and Lloyd (WO2004056017) describe a TDM system that uses a broadband light source, which is pulsed and gated with a semiconductor optical amplifier. Wavelength determination is accomplished with an optical filter having known transmission characteristics, or alternately with an optical spectrum analyzer or a wavemeter. Whilst the use of a semiconductor amplifier results in a larger optical power, the combined cost of the source, optical amplifier, and wavelength measuring apparatus is still important.
One difficulty when using a broadband optical source is that the optical power is distributed over the entire source spectrum. Thus the actual power reflected by an individual FBG sensor is only a small fraction of the total source power. In actual fact, most of the source power is wasted, as it is not reflected by any sensor. Typical broadband sources, such as light emitting diodes, or superluminescent diodes, only have a total power of the order of 1 mW, after coupling into a single-mode optical fiber. The low power of the reflected signal makes it difficult to obtain a large enough signal-to-noise ratio. The other difficulty with broadband sources is that the number of spectral windows is still limited by the total spectral width of the source.
All in all, most prior art suffer from an additional drawback: the cost of the system. Optical amplifiers, fiber interferometers, or spectrum analyzers are relatively expensive devices. The high cost of these systems has so far impeded their widespread adoption. One other factor affecting the cost of the apparatus is that it is an all-or-nothing instrument. That is, the instrument is capable of measuring a high number of gratings at a given cost, and there is no way of paying less if the number of sensors required is smaller than the ultimate capability of the instrument.
There is therefore a need for improved optical fiber sensor arrangements.